1. Field of the Invention
The present invention relates to a process for separating arsenic from acid solutions. More particularly, the invention relates to a process for separating arsenic from sulfuric solutions of copper and other ions.
Almost all commercial copper is produced by electrolytic refining of a raw material comprising 99.5% copper and further containing arsenic; many other impurities are also present. The raw material (also known as "blister") is suitably smelted and recast into plates to form the anode of an electrolytic cell for the refining process; the cathode is typically a copper plate or, in the most modern plants, a stainless steel or titanium plate. The electrolyte is usually a solution of cypric sulfate, containing much free sulfuric acid.
When electric current is applied during refining, copper dissolves from the anode and deposits at the cathode. Some of the impurities of the anode are insoluble and form the so-called "anode mud", while other impurities (elements less noble than copper) also dissolve electrolytically. One of these latter impurities is arsenic, which thus builds up in the electrolyte, from which it must be removed in order to avoid copper contamination at the cathode.
There are also other metallurgical processes in which the control and the removal of arsenic are very important. For instance, similar problems exist in the case of the recovery of copper by electrolysis with insoluble anodes from residuals, i.e., the copper scums coming from purification stages in the primary metallurgy of lead.
In all cases, the input-output equilibrium of arsenic (and of the other impurities) must be controlled. A well-known technique is to bleed part of the copper electrolyte which is then sent to multi-stage electrowinning cells in which arsenic is eliminated as a copper-arsenic alloy; however, the electrolysis of a solution poor in copper and rich in arsenic causes the hazard of generation of arsine, a very toxic gas, which therefore requires particular facilities and precautions. Furthermore, the recycling of the product alloy sometimes presents technical and economic problems.
Therefore, it is particularly interesting to have an easier process for the selective removal of arsenic from the copper electrolyte or from strongly acid solutions, and solution extraction has been proposed.
2. Discussion of Related Art
Methods wherein arsenic is extracted from copper electrolyte with suitable organic solutions such as solutions containing tributylphosphate as the main active component are known. From such solutions arsenic is extracted with water or acid or alkaline aqueous solutions, and separated by precipitation as the corresponding sulfide, using hydrogen sulfide or alkaline sulfides, or as arsenic trioxide, using sulfur dioxide or sulfites.
DE-OS No. 2.603.874 discloses the separation of arsenic from copper in sulfuric acid solutions by extraction with hydrocarbon solutions containing 50-75% of tributylphosphate and 5% or less of a high-boiling alcohol, the function of which is stated to prevent the formation of a third phase when the concentration of arsenic in the organic phase increases.
DE-OS No. 2.615.638 discloses the similar use of a trialkylic, triarylic or triarylalkylic phosphate, preferably tributylphosphate, diluted with a hydrocarbon and containing smaller quantities of isodecyl alcohol, which has the function of emulsion inhibitor.
GB-PS No. 1.551.023 discloses the comparable use of a hydrocarbon solution containing 40-75% of tributylphosphate and up to 15% of a quaternary ammonium salt, preferably methyltricaprylammonium chloride or a mixture of methyltri(C.sub.8 -C.sub.10)alkylammonium chlorides.
In all the three cited specifications, tributylphosphate, a quite toxic substance, is used in considerable quantities; furthermore in GB-PS No. 1.551.023, the addition of a quaternary ammonium salt causes a further increase in toxicity in the extracting organic mixture. Furthermore, in these specifications, the affinity of the extractant for arsenic is moderate, and, as a result, many extraction stages with a high ratio of organic/water are required.
Similarly, the capability of polyphenols having two hydroxyls in vicinal positions to form chelate complexes with arsenious acid in strong mineral-acid solutions is well-known. Such complexes with pyrocatechol and with pyrogallic acid, formed by the reaction of a molecule of arsenic or arsenious acid respectively with three or two molecules of the polyphenol, were carefully examined.
SU-PS No. 765.402 discloses the separation of arsenic from acid solutions using a resin comprising the polycondensation product of pyrocatechol and formaldehyde. However, such resins typically have a poor affinity for arsenic due to the necessity of arranging two or even three phenolic groups to allow bonding with the arsenic according to the geometric configuration that the complex must assume; such a possibility is prevented by the three-dimensional structure of the resin and thus the affinity of the resin for arsenic is reduced by one order of magnitude. This condition is common for all cases in which, to bind the metal, more than one chelating molecule is necessary. Thus, as a practical alternative, the liquid/liquid extraction remains, in which organic chelating agents, which are practically water-insoluble are used as extraction agents.